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THE PRESENT STATUS OF THE SWINE
INDUSTRY OF THE UNITED STATES AND STRATEGIES FOR FUTURE IMPROVEMENT
L. L. Christian
Iowa State University
Ames, Iowa,U.S.A.
Introduction
The pork industry of the United
States continues to change, but at a rata that appears to be increasing. The
number of farms that raise pigs has declined dramatically in the past 20 years.
Nationally, in 1991 there were only 30% as many farms with hogs as there were in
1970 (Figure 1). In the midwestern states and in Iowa in particular, the
traditional center of hog production, has fared better than the national average
but have lost 60% of their producers over this period. But, while the number of
hog farms has declined, total numbers of hogs produced has remained stable. The
number of hogs per farm has more than doubled in Iowa and the other midwestern
states. In the state of North Carolina on the eastern seaboard an eight-fold
increase in number of hogs per farm has occurred over this 20-year period.
The distribution of hogs per farm
has also changed over this time frame. In 1991 about 904 of U.S. hog farms had
less than 500 head inventory. But, these small farms accounted for only 32% of
the national total (Figure 2). Conversely, farms with over 1000 head inventory
represent over 44% of the nation’s production and less than 5% of the hog
farms. It is expected that soon to be released 1992 Census of Agriculture data
will reveal continued growth of farms marketing over 5000 head and a further
Decline in the portion contributed by the small farm group. It is anticipated
that the over 5000 head group will account for over 25% of all the hogs marketed
in our country.
Why are these changes occurring?
As was noted by Lawrence (1992),it is because the general structure of the
marketing channel is changing. In the past farmers finishing pigs also farrowed
them, raised replacement gilts and raised corn. Then, there was a distinct
break between production and the packing, processing, wholesale distribution
and retail segments. The industry relied on price signals sent through the
various segments of the industry and an adversarial relationship existed between
producer and packer.
But now the marketing channel is
changing to a more totally integrated system where the separate entities are
linked together permitting greater flow of information and direct signals
between the segments. In many cases the segments are controlled by the same
management. Several essentials are necessary for the producer - centered system
to persist according to Ginder (1989).
He lists these foundation variables
to be the ability of independent producers to finance their production, for
them to be the low-cost producers, for them to be able to sell in any market
without substantial discounts and for them to have full access to technology. It
is apparent that in the U.S. society today these essentials no longer exist to
the extent they did in the past.
Figure 1
Source: USDA Hogs & Pigs
Percentage Change In
Number of
Hog Farms: 1970 = 100
|
Percent of Operations
and Inventory
by Farm Size, United
States 1991
|
The
Seedstock Industry
The seedstock industry of the U.S.
has changed to service the large more specialized units. Not only have private
breeders increased the size of their units but they have taken on more than one
breed to serve the entire needs of their commercial customers. Large corporate
breeding companies have developed and/or moved operations to the U.S. to enter
this market. Table 1 presents results of a comprehensive survey conducted by
michigan State University in 1988. Approximately 304 of the paternal line
seedstock was being supplied by the three major corporate seedstock companies
operating'in the country. Since that time many more have developed and expanded
their production. Outside companies entering the market include Cotswold,
National Pig Development, Newsham, and Seghers. Of over 15DO market pigs
voluntarily submitted by commercial producers to a state pork producer owned
station at New Hampton, IA in 1992-93 approximately 454 were sired by
company-produced boars, 384 were sized by purebred boars and 174 by crosses
among the pure breeds. Hence, it is apparent that in the U.S. corporate-owned
breeding companies are gaxnering an ever increasing share of the seedstock
market.
Table 1.
Source for Acquiring Replacement Breeding Stock for Paternal (Terminal) Linesa
No.
Head
Produced/year |
PIC |
Farmers
Hybrid |
DeKalb |
Babcock
Swine |
|
------------------Percent
of Market Share------------------ |
Less
than 999 |
0.2 |
1.1 |
0.3 |
0.0 |
1,000
to 2,999 |
2.4 |
3.7 |
2.0 |
0.2 |
3,000
to 4,999 |
1.3 |
0.7 |
1.3 |
0.0 |
5,000
to 9,999 |
1.2 |
0.7 |
2.6 |
0.0 |
More
than 9,999 |
2.8 |
1.2 |
1.6 |
0.0 |
TOTAL |
7.8 |
7.4 |
7.7 |
0.2 |
a1988
MSU Survey of Swine Breeding Systems in the U.S.
Regardless of who supplies the
seedstock t.o the coaaaarcial sector, the basic concepts necessary to create
genetic change at the production level are the same. While modern techniques of
genetic engineering may someday alter the genome used by the pork industry in
a practical way, there are presently only two approaches available to the
pork producer to bring about genetic change--selection and choice of breeding
system. This discussion vill focus on each of these as they are being used to
accomplish genetic improvement of swine in the United States.
Relative
Importanae of Traits
Careful examination of the
performance traits test exert the greatest impact on praduction costs of U.S.
producers (Table 1) reveal that reproduction and maternal performance traits
should be of greatest concern. Carcass premiums offered by some companies for
laaner pigs have increased in recent years and have significantly
increased the relative economic value for body composition. The growth
rate traits of average daily gain and days to 105 kg have the least direct
effect on net income.
The economic values used in this
table are for a full unit of change, either one pig per litter, one kg of litter
weaning weight.', one day change in days to 105 kg, one mm change in backfat
probe, one kg per kg of gain change in feed efficiency and one kg per day change
in daily gain. These values multiplied by the standard deviation put the
economic value for each trait on a weighted basis.
Table 2.
Parameter estimates and economic values for swine traits. (U.S. dollars)
Trait |
Heritability |
Standard Deviation |
Economic Value/unit |
Economic Value |
Relative
Economic Value |
Number
born alive (L) |
.15 |
2.50 |
12.00 |
30.0 |
15.0 |
Adjusted 21-day
litter weight (W) |
.15 |
9.00 |
1.5 |
13.5 |
6.7 |
Days
to 105 kg (D) |
.35 |
13.00 |
-.15 |
2.2 |
1.1 |
Backfat (B) |
.40 |
.50 |
-6.00 |
3.0 |
1.5 |
Feed
efficiency (F) |
.30 |
.25 |
-13.00 |
3.2 |
1.6 |
Averaige daily gain (G) |
.40 |
.09 |
22.0 |
2.0 |
1.0 |
New Analysis
Procedures
The Mixed
Model Approach
The year 1990 will be remembered as
a time of major significance to swine seedstock improvement. The first national
across herd genetic evaluation was completed for a major swine association, the
American Yorkshire Club. This major event will soon be mimicked by all of the
major breeds. Through January of 1993 over 60,000 pig records and 110,000 sow
records had been processed by the American Yorkshire Club's across herd analysis
(table 3). The Landrace breed has compiled its first across herd analysis of
both maternal and pig data and in Juae of this year the Hampshire and Duroc
breeds will release their first across herd analysis results which will include
over 25,000 sow records and 20,000 pig records in each breed.
There were at least three major
factors that contributed to this important historic development. First was the
development of the Best Linear Unbiased Prediction (BLUP) procedure by C. R.
Henderson at Cornell University (1973) and it adaptation to swine data. This
methodology overcame the problems of unequal numbers per group (herd, sire
progeny, etc.) and the difficulties of removing fixed effects (year, season,
contemporary group, etc.) from the random genetic effects of concern. This
approach permitted improved accuracy of selection by including family and prior
statistical information in selection (Gibson and Smith, 1986). A second
important development was the availability of new computing and data processing
capabilities that reduced the time delay from the completion of the record to
evaluation and selection. A third important development was the discovery and
adaptation to farm animals of sophisticated ultrasonic procedures which permit
accurate assessment of composition in the living animal.
BLUP is now recognized as the most
effective method of genetic evaluation. The procedure uses not only the
individual’s own record, but that of all of his relatives to predict an Expected
Progeny Difference (EPD) for each trait evaluated. EPD is the predicted
difference in performance of future offspring of a sire or dam relative to that
of parents of average genetic merit. In short, EPD evaluates according to their
genetic value as parents.
There are a number of features of
BLUP that are responsible for its rating as the best genetic evaluation system.
Also, there are a number of sources of bias that can lead to inaccurate genetic
evaluations. Differences between herds and groups within a herd in environment,
ration content, housing, and other feeding and management procedures will affect
the way animals perform. The genetic merit of the animals to which an individual
is mated will cause bias. The BLUP statistical procedures simultaneously
estimate differences associated with herds and contemporary groups as well as
adjust for the genetic merit of mates. The result is an unbiased estimate of the
genetic value of sires, dams, and pigs. The multiple trait BLUP procedure used
by the Swine Testing and Genetic Evaluation System (STAGES) includes all
relatives in each participating herd. Through the use of this informatiqn and
the genetic correlations between traits, the EPD is the most accurate possible.
Table 3.
Yorkshire Across-Herd Analysis Summary, January 1993.
Number of |
Growth Data |
Maternal Data |
Herds |
51 |
664 |
Sires |
952 |
6200 |
Active
Sires@ |
773 |
2458 |
Dams |
6273 |
23213 |
Pigs
(or Sows) |
63777 |
50686 |
Sow
Records |
-- |
110603 |
Caution must be exercised in
conducting these programs in order to prevent bias. Preferential treatment of
sire or dam progenies will permit them to appear better than they are
genetically. A second concern is honesty and integrity in collecting records.
This is of concern since operators may choose to collect records themselves or
to have them obtained by farm employees in order to reduce costs. To ensure
credibility these records must be obtained carefully, perhaps by special
technicians. Premature culling of unpromising young animals of certain sires can
also lead to bias. In spite of these limitations, this procedure still excels
all other pmeedures used to date for effective genetic evaluation.
On-the farm
Programs
The BLUP procedure adapted for use
in swine is called the Swine Testing and Genetic Evaluation System (STAGES). It
was developed by Purdue University in conjunction with the USDA, the National
Association of Swine Records and others. It is a well developed on-the-orem
pxogram that captures‘ individual performance and combines it with information
on miativea. ’Ihe STAGE 6 step is now in place and involves across-herd
evaluation (Stewart et aL, 1991). The merits of this and other on-the-hrm
programs include:
1. Whole herd
testing is possible.
2. Low test
cost per animal.
3.
Reproductive and maternal traits can be measured (Sow Productivity Index, SPI).
4. Large
numbers of progeny per sire permit accurate sire evaluation.
5. The system
permits selection priorities that are unique to the goals of the individual
herd.
6. Indexes may
be developed for specialized general, maternal or paternal lines.
Maternal
Traits
In the past, it was believed that
selection for lowly heritable, sex-limited sow productivity traits would not be
effective. A summary of heritabilities of these traits appears in Table 4.
Several recent findings, however, give us renewed optimism. Hyperprolific sow
line development has shown that where population size is large, selection has
been effective. Secondly, theoretical projections of Avalos and Smith (1987)
show that annual changes of up to 1/2 pig per litter are possibles through
family selection. STAGE 6 uses aU of the family in$wnation in an optimum manner
and should be effective for the lowly heritable sow pmdvctivity traits (Belonsky
and Kennedy, 1988).
Table
4. Literature averages for heritabilities of reproductive traits.
Traits |
No. Observations |
Estimate |
Age at
puberty |
8,119 |
.32 |
Ovulation rate |
6,088 |
.39 |
No.
born per litter |
24,137 |
.10 |
No.
born alive per litter |
138,248 |
.07 |
Survival to weaning |
78,738 |
.05 |
Rebreeding interval |
135,569 |
.06 |
Litter
birth weight |
3,955 |
.29 |
21-day
litter weight |
76,335 |
.15 |
From:
Lamberson (1990).
Evaluation of repeated records of
the same sow have revealed a high genetic correlation (r=.96) between repeated
records for SPI traits (Johansson and Kennedy, 1985), thus permitting the
evaluation of these traits as sing1e rather thw multiple traits. In addition,
the conclusion that maternal genetic effects are small for sow productivity
traits (although slightly negatively associated with direct genetic effects) are
highly variable and can be ignored with little loss of information (especially
if litter sizes are balanced at birth). This permits a rather simplified
approach to genetic evaluation of these traits. However, maternal and
environmental factors common to littermates tend to make littermates more alike
than expected from their genetic relationship alone (although small for most
traits) and is accounted for in the STAGES model for these traits.
The STAGE 6 program computes EPDs
for the two maternal traits: number born alive per litter (NBA) and 21-day
litter weight (L%21). These EPDs are combined into a SPI that is expressed as a
deviation from the average of the base population in dollar units. An EPD for
NBA is expressed in number of pigs. Daughters of an individual with an EPD of .4
for this trait would be expected to farrow .4 more pigs than the daughter of the
average sow. An EPD of +5 for LW21 would mean 5 more lbs of 21-day litter weight
were produced by daughters.
The SPI is expressed as the sow’s
EPD for sow reproduction. Former SPI programs were phenotypic values that only
measured a sow’s individual reproductive records. By induding the genetic
performance of relatives, the potential for rapid progress is improved
dramatically. The EPD is 1/2 of the sow’s genetic superiority above her
contemporaries (i.e., the amourit she will transmit on the average to her
progeny). For example, if a sow’s EPD is 105, each of her daughters would be
expected to produce litters worth $5 more at 21 days San those of a sow with an
EPD of 100.
Performance and Carcass Traits
These traits are of economic
importance and have sufficient heritability to respond to selection (Tables 5
and 6). The genetic correlations among them are generally small or strongly
favorable. The across herd STAGES program also involves the collection and
analysis of on-farm postweaning growth and backfat information. The two traits
involved are:
Adjusted Days to 105 kg (D105)
- A measure of growth rate by using off-test age and weight. The recommended
procedures are outlined by the National Swine Improvement Federation (NSIF,
Weber, 1987). An individual’s own performance and that of his relatives are used
to produce his EPD for days to 105 kg, a prediction of how this individual's
progeny are expected to perform relative to the average of the genetic base
population. For example, an EPD of -6 for D105 indicates that pigs produced by
this individual would reach market weight (105 kg) 6 days earlier than those by
the average of his breed.
Adjusted Backfat (BF)
- The backfat of an animal adjusted to 105 kg. This procedure also follows NSIF
Guidelines (Weber, l987). Adjusted values are converted to an EPD for backfat
which is a measure of the expected superiority (or inferiority) of his progeny
relative to that of an average boar. These EPDs are a function not only of the
boar’s own backfat but alse that of all of his relatives both within and across
herds. A boar with an EPD BF of -1.0 would be expected to produce progeny that
average 1 mm less in backfat than those from the average boar of the breed.
New developments in ultrasonics
have increased the accuracy of assessing backfat in live animals. Machines with
the third fat layer measurement provision permit the operator to easily detect
and accurately measure this additional layer that is commonly present. Real-time
machines, though expensive, are extremely accurate in measuring backfat and are
an improvement over previous devices for estimating loin muscle area.
Correlations between live and carcass values of approximately .90 for backfat
and .80 for loin muscle area have been reported with the latest versions of this
equipment (Christian and Moeller, 1990).
Table 5. Heritabilities of growth,
feed and backfat traits and their genetic (6) and phenotypic (P) relationships
to percentage lean.
Trait |
h2 |
% Lean (G) |
% Lean (P) |
ADG |
.30 |
-.15 |
-.11 |
Days |
.25 |
.10 |
.10 |
Backfat |
.41 |
-.85 |
-.71 |
F/G |
.30 |
-.43 |
-.25 |
AFI |
.24 |
-.25 |
-.20 |
From: Stewart
and Schinkel (1990).
Table 6. Heritabilities of carcass
characteristics and their genetic (G) and phenotypic (P) relationship to
percentage lean.
Trait |
h2 |
% Lean |
% Lean |
BF10 |
.52 |
-.87 |
-.81 |
LMA |
.47 |
.65 |
.62 |
Dress
% |
.30 |
-.10 |
.00 |
Length |
.56 |
.18 |
.10 |
% Lean |
.48 |
.48 |
--- |
Fmm: Stewart
and Schinkel (1990).
Measurement of feed conversion is
considered to be optional in improvement programs because research reports
indicate low realized heritability for this trait. Indirect selection for
leanness and rapid growth in several studies has resulted in as much or more
improvement in feed efficiency than has direct selection <able 7). This is
fortunate since Sew breeders, because of the high costs involved, have
facilities to measure feed intake on the small, genetically related groups
necessary to yield meaningful data for selection.
Table 7.
Phenotypic correlations of various traits with efficiency of lean gain (ELG).
Trait |
ELG |
G |
.24* |
F/G |
.66** |
% LC |
.28** |
LC
Gain |
.46** |
*P < 0.05.
**P < 0.01.
From: Bereskin
and Davey (1976)
Selection
Indexes
Selection indexes reported in
STAGES are optimized according to the economic values associated with costs of
production and value of market pigs in a typical midwestern farrow-to-finish
pork production facility. Four indexes are calculated. SPI (mentioned earlier)
ranks individuals on NBA and LW21 only. Terminal Sire Index ranks the animals on
D 105 and BF only. The General Purpose Index (GPQ ranks the animals on a
combination of growth and maternal traits, and is recommended for breeds and/or
lines used in rotational crossbreeding programs. Maternal Line Index (MLQ also
utilizes the EPD of both growth and maternal traits but is optimized for a
maternal line. All indexes are scaled so that the average parent has a value of
100 and superior parents have values greater than 100. Which index is most
appropriate depends on the traits measured and the breeding objective. In most
commercial pork production systems following a terminal or rotaterminal system,
replacement gilts should be selected on the MLI and replacement boars selected
on TSI.
Accuracy
An accuracy value is listed with
each trait in the STAGES output. Accuracies range from .01 gow) to .99 (high).
Accuracy is an indicator of precision with which the EPD is estimated (i.e., the
level of confidence that the predicted value of the EPD is near the true genetic
value of that sire).
When accuracy is high, the EPD is
reliably predicted. If the sire is used, the breeder can be quite confident that
the mean offspring performance will be near the predicted level. If the accuracy
is lower, the mean performance of the offspring may vary more from the
prediction. It is very important to understand that the error of prediction of
EPDs is unbiased. That indicates that the EPD has an equal chance of being
either over or under in predicting the actual performance, but on average is
correct.
If, in deciding between bvo st with
ami1ar EPDs, you want high predictability, then select the sire with the highest
accuracy. You can be confident that the mean of his offspring will be nearer the
estimate than will that of a sire with lower accuracy. In this case, you haved
the possibility of selecting a sire whose offspring will perform much poorer
than predicted. But, you have also negated the chance of selecting a sire whose
offspring will be much better than expected. If, on the other hand, you are
willing to take a greater risk in an attempt to select an outstanding sire, then
choose the sire with the lower accuracy but higher EPD. This sire has a greater
chance of producing offspring that will perform better than expected. Of course,
there is an equally greater risk of failure.
Selection decisions should be based
on the EPDs first, then use accuracy to decide among those sires with simBar
EPDs. Accuracy is determined by the amount of information available to predict
the EPD and is not affected by the level of performance of those records. (The
EPD is the indicator of the level of performance.) As more records on more
relatives are available, the accuracy of the prediction increases. But, it is
not simply a matter of getting many records. Accuracy is also related to how
many different herds, contemporary groups, relatives, and other sires and dams
are included in the data set. Therefore, sometimes'sires with fewer offspring
wi11 have a higher accuracy because his relatives are spread over more herds,
groups and mates. In addition, a major determiner of accuracy is whether the
individual itself has a recorded performance or not.
Central
Testing
Since 1956 central testing has been
a favorite selection technique used by U.S. breeders. Records have provided a
benchmark for assessiqg change over time (much of which, we hope, has been
genetic). This approach has aided in identifying superior herds and individuals
and has provided the small breeder the opportunity to advertise his stock and
assess the relative merit of his animals relative to that of his competition.
Stations have provided a "show window" for the swine industry and have
contributed greatly to the education of both seedstock and commercial producers
concerning genetic principles and breeding programs.
Participation and interest in
central testing has dwindled somewhat in recent years (table 8). Performance
summaries comparing 1991 results with those from 10 years earlier reveal that
progress has been made in growth rate and feed conversion but that backfat has
remained rather constant (tables 9-12). No single reason is considered to be
responsible, but surveys have mrealed a number of possible causes:
1.
Concern that pretest
environment iafluences performance on test.
2.
Health concerns
reduce the number of commercial producers wiBing to iatmduce animals to their
herds from multi-source testing facilities thus buyer demand has been reduced.
3.
Larger sized
commexeial herds desire more boars of a given bxeed or line than are available
from a single station.
4.
Increased testing
costs.
5.
Reduced faith in the
accuracy of ultrasonic estimates of carcass traits.
A number of possibilities exist for
overcoming the above concerns. These include running all pigs through a
medicated early weaning program at a common site prior to being placed on test,
testing larger groups of contemporary boars of a single breed at one time, and
using real-time ultrasonics to evaluate backfat and loin musde area. It is hoped
that faith and partici jation in central testing can be restored since it
provides a basis for comparisons between herds, something especially important
for the small herd, and that adds credibility to the on-the-farm records of
participating producers. Recently BLUP procedures have been used to develop EPDs
for animals tested in all stations across the U.S. Utilization of this
information will improve the accuracy of selection and provide a valuable
service to the private breeder in locating genetically superior stock.
Because of reduced interest in boar
testing, several stations are now devoting some or all of their space to the
testing of market pigs. The purpose of some of these tests has been to provide
accurate carcass information on sib and progeny groups without the need of
commingling the breeding animals themse1ves. Others are designed to simply
evaluate the products of various terminal breeding programs. The Pork Challenge
Test, conducted in each of the last three years in conjunction with the World
Pork Expo, is an example of a large scale test for assessment of terminal market
hogs. This test has provided a fair and unbiased comparison of germ plasm
sources available to the commercial hog producer of the U.S. (table 10). Not
only did this test compare terminal market hogs derived from purebred and
corporate seedstock sources for production and quantitative carcass traits,
other traits have been compared as well. The cholesterol and lipid content of
muscle from these pigs not only differed considerably between the various
crosses, but were much lower than values reported earlier. These findings point
to possible genetic variation in these qualitative traits and to the nutritious
and healthful aspects of lean pork. This test provides a model that could be
copied in tests of a national scope to evaluate pure lines. Such a test is now
being conducted under the supervision of the National Pork Producers Council.
Table 8. Boar
Testing Survey Performance Summary
|
Number Tested |
1991 |
1981 |
Berkshire |
95 |
61 |
Chester White |
143 |
152 |
Duroc |
883 |
1,915 |
Hampshire |
564 |
671 |
Landrace |
215 |
281 |
Poland
China |
38 |
56 |
Spotted |
177 |
461 |
Yorkshire |
1,016 |
1,416 |
Total
or Avg. |
3,131 |
5,013 |
Table 9. Boar
Testing Survey Performance Summary
|
Average Daily Gain |
1991 |
1981 |
Berkshire |
981 |
903 |
Chester White |
981 |
854 |
Duroc |
1044 |
953 |
Hampshire |
985 |
935 |
Landrace |
1003 |
917 |
Poland
China |
922 |
908 |
Spotted |
958 |
922 |
Yorkshire |
1035 |
953 |
Total
or Avg. |
1017 |
944 |
Table 10. Boar
Testing Survey Performance Summary
|
Feed Efficiency |
1991 |
1981 |
Berkshire |
2.52 |
2.64 |
Chester White |
2.49 |
2.63 |
Duroc |
2.37 |
2.46 |
Hampshire |
2.41 |
2.49 |
Landrace |
2.38 |
2.64 |
Poland
China |
2.55 |
2.78 |
Spotted |
2.48 |
2.64 |
Yorkshire |
2.40 |
2.53 |
Total
or Avg. |
2.41 |
2.52 |
Table 11. Boar
Testing Survey Performance Summary
|
Average Backfat |
1991 |
1981 |
Berkshire |
20.3 |
21.1 |
Chester White |
20.3 |
20.3 |
Duroc |
19.6 |
19.3 |
Hampshire |
18.5 |
18.5 |
Landrace |
18.8 |
20.3 |
Poland
China |
20.3 |
22.3 |
Spotted |
20.0 |
19.8 |
Yorkshire |
19.8 |
19.6 |
Total
or Avg. |
19.6 |
19.6 |
Table 12. Boar
Testing Survey Loin Eye Estimates
|
Loin Eye Area |
1991 |
1981 |
Berkshire |
36.1 |
35.5 |
Chester White |
37.5 |
34.1 |
Duroc |
36.2 |
35.2 |
Hampshire |
38.1 |
36.9 |
Landrace |
37.0 |
34.6 |
Poland
China |
36.8 |
36.6 |
Spotted |
35.9 |
34.9 |
Yorkshire |
35.9 |
34.4 |
Total
or Avg. |
36.6 |
35.2 |
Table 13.
Efficiency of lean gain by breed of sire in the Pork challenge test 1988-90.
Breed
of Sire |
1990
No. of Pens |
1990
Feed/Lean |
Total
No. of Pens |
Total
Feed/Lean |
Standard
Error |
Babcock |
2 |
8.09 |
4 |
8.27 |
±.181 |
Berkshire |
3 |
8.57 |
6 |
8.56 |
±.148 |
DeKalb
77 |
2 |
8.28 |
10 |
8.13 |
±.114 |
DeKalb
88 |
2 |
8.25 |
|
|
|
Duroc |
4 |
8.22 |
13 |
8.26 |
±.100 |
Duroc-Hamp(F1) |
2 |
7.99 |
8 |
8.34 |
±.127 |
Farmers Hybrid |
4 |
8.20 |
12 |
8.36 |
±.103 |
GIS |
2 |
8.32 |
|
|
|
Hampshire |
5 |
7.82 |
14 |
7.94 |
±.097 |
Lucie
Hybrid |
2 |
8.38 |
4 |
8.56 |
±.187 |
PIC HY |
3 |
7.68 |
7 |
7.78 |
±.136 |
PIC
L-26 |
2 |
7.58 |
9 |
7.72 |
±.121 |
Poland
China |
|
|
4 |
8.66 |
±.184 |
Yorkshire |
2 |
8.48 |
8 |
8.67 |
±.129 |
AVERAGE |
|
8.12 |
|
8.23 |
|
This program utilizes Al and MEW:.
lures (Figure 3) to reduce health risks associate with the commingling of
animals from many herds. Cooperating producers will receive semen representing
at least three different terminal lines each breeding period to breed to their
saws. Litters resulting from these matings will be sampled (1 or 2 pigs).
Selected pigs will be entered in Performance MEW facilities. Pigs will go from
Performance MEW facilities to central test facilities for growth rate and feed
efficiency evaluation. Carcass traits will be measured at a packing plant and
loin muscle samples evaluated for muscle quality at a research laboratory.
Breed associations or breeding
companies will nominate their breeds or lines and sires to the Program Director.
A program goa1 is to have many sires per line but few progeny per sire. Soars
miintained in facilities raeeting the "Health Guidelines for Boar Studs" will
have semen collaAed in these approved locations. The GPC will approve semen
collection, extension, and distribution practices. Boars sampled will be less
than 15 months of age. Each boar will be given a coerfidenha1 code number.
All the sows during a breeding
period within each cooperating herd must be genetically uniform, such as F1
cmssbreds, PIC Camborough, etc. If possible, parity of each sow used in this
program should be recorded to account for litter parity effects. If sows are
clearly identified within a herd by parity and genetic type, mene than one
genetic type of female may be used in that herd. There must be females of each
parity of each genetic type used in the program in that herd. Two doses of semen
per sow wi11 be provided.
Figure 3. Terminal Line Program of
National Genetic Evaluation.
The number of approved commercial
producers willing to use artificial insemination sow breeding methods will be
established and a schedule for semen distribution established for each breeding
period. Records of mating and sow pregnancies will be reported within 60 days of
breeding to the Project Manager. Cooperating producers will receive a $20 per
sow recording fee when breeding records are reported to NPPC.
An impartial person designated by
the Project Manager will select barrows and/or gilts from each litter at 10-20
days of age. Selected pigs should represent the average of their litter. One or
two pigs per litter will be sent to a Performance MEW station depending on
program needs. Producers will receive the price of a 18 kg feeder pig for each
selected pig.
As pigs finish the MEW program,
they will be sent to a testing facility for growth and feed efficiency
measurement. Pigs will be penned by linage of sire. Structural soundness will
also be recorded. These records will be reported to the Project Manager.
As pigs reach 109 kg they will be
sent to a cooperating packer. Traits of backfat depth, him muscle area or
trimmed musde weight, carcass weight and carcass length will be measured
according to these procedures. Visual scores of loin muscle quality will be
assigned. These records will he reported to the Project Manager.
Two loin chops between the 10th and
12th rib will be removed from each carcass and dehvered to a research
laboratory. One chop will be used for laboratory tests of muscle quality and
nutritional content (lipid, etc.) and the other chop will be cooked to 160
degrees and measured for tenderness, lipid content, sensory panel, cooking loss,
moisture content, and any other trait approved by the GPC. These records will be
reported to the Project Manager.
It is important that pork producer
associations at both the state and national levels as well as university
extension personnel continue to support performance and progeny testing
programs. Such participation is justified since emphasis is only on the
economically important traits and the services are available to all seedstock
producers regardless of breed or company affiliation. The benefactors of these
programs are the consumers who ultimately should enjoy a better quality product
at a lower price, and the commercial pork raiser who is aided in his search for
superior stock as a result of this service. Association and university
leadership assures control over publication of results and permits smooth
modification of testing procedures should it be warranted by research findings.
Population
Size
Selection acmss herds is very
important. This is especially true if herd sizes are relatively small.
Population size has a major effect on rate of genetic gain. A fifty sow breeding
herd can achieve only about half the genetic gain possible in a 300 sow herd
(Kennedy, 1989). Most purebred seedstock producers have herds of much fewer than
300 sow', but numbers that large or )arger are common in pure lines of most of
the major breeding companies. To remain competitive, individual breeders must be
part of a population much larger than their own herd by having access to genetic
evaluations that permit selection across herds, such as through EPDs. Although
the number of recorded litters of the various breeds has dwindled in recent
years, all eight of the major breeds present in the U.S. are of sufficient size
(table 14) to create near maximumprogress if across herd selection is practiced.
This requires a breeding structure that facilitates such comparisons. That is
why use of test stations and AI are important to genetic gain in both the
individual breeder’s herd and in the whale population. These provide most of the
genetic links that can give the individual seedstock producer the advantage of
large effective population size. Because of health concerns, a decreasing number
of live animals are transferred among breeder herds. Some procedures such as
embryo transfer and cesarean section are very expensive. AI provides a
relatively economic means for breeders to link their herds together genetically
so as to create a larger effective population size. This is important for
improvement of all economic traits, but is particularly important for genetic
improvement of sow productivity traits.
Table 14.
Litter Recording Summary By Breed
No. of
Litter
Recorded |
1982 |
1987 |
1992 |
Berkshire |
3,245 |
2,091 |
2,137 |
Chester White |
6,436 |
6,439 |
6,358 |
Duroc |
31,975 |
22,000 |
20,248 |
Hampshire |
17,711 |
18,160 |
20,824 |
Landrace |
6,170 |
3,215 |
4,589 |
Poland |
3,010 |
2,411 |
1,766 |
Spotted |
12,600 |
7,906 |
7,469 |
Yorkshire |
25,581 |
22,246 |
26,659 |
TOTAL |
106,728 |
84,468 |
90,050 |
Visual
Traits
Caution should be exercised to
ensure that breeders do not place too much emphasis on "indicators of
performance" instead of measuring performance itself. Some traits, such as
structural soundness, length of body, underlines, general conformation and
presence of physical defects, can only be evaluated visually on the live animal.
Appraisal of animals with outstanding performance will train the keen observer
to make selections among animals lacking performance figures or to make
meaningful selection among animals from different herds. Biased or erroneous
measurements can sometimes be uncovered by a watchful eye. Performance records,
in addition to "eyeball appraisal", should be better than either method by
itself.
In addition to the maternal and
growth traits, it is essential that the performance testing program include
careful assessment of animals for structural soundness and freedom from stress
susceptibility. Findings at Iowa State University and elsewhere reveal clearly
that leg structure is moderate in heritability and will deteriorate if not
constantly monitored (Rothschild and Christian, 1988).
Underline soundness is also of
importance although probably overemphasized in selection. Teat number is
moderately heritable in swine (most estimates suggest it to be approximately 30%
inherited). Whether gilts with less than 12 prominent nipples are kept should
depend on the level of production achieved in the herd. It seems rather
senseless to discard the leanest probing, fastest growing gilt in the herd if
only eight pigs are normally raised per litter. Number of functional nipples
rarely constitutes the limiting factor in number of pigs raised.
Inverted nipple problems are
inherited as a recessive trait but at least two pairs of genes seem to be
involved. Although it is a rare exception when the number of inverts is greater
than two on a gilt, invariably these are located in the prime locations and
hence animals showing this undesirable condition should be culled. A boar with
undeveloped or inverted nipples can spread this problem through a herd rapidly
and reduce the selection pressure achieved in other economically important
traits.
Porcine Stress Syndrome (PSS)
PSS and its associated problem,
pale, soft exudative muscle USE), continues to plague the pork industry.
Detection methods are available that can accurately determine the various stress
genotypes. These will likely be used by the industry if PSS become a more
serious problem, and if muscle quality (PSE) detection devices are developed
that can accurately determine potential PSE carcasses during the slaughter
process and at line speed. If such carcasses can be identified and are
discounted, greater producer interest in PSS and PSE elimination will develop,
Because susceptibility to the PSS
is definitely under single gene, autosomal recessive contml, problem herds
should be'carefully monitored by halothane screening and/or DNA testing to
reduce the frequency or eliminate the PSS gene (Christian, 1989). The DNA test
is considered tp be a major breakthrough that will have a tremendous impact on
the use of this gene.
The DNA
stress test
Through its use the presence or
absence of this mutated gene is readily detected. Five to 10 ml of blood are
drawn into a heparin or EDTA treated tube and transported to a diagnostic
laboratory where DNA is extracted. The desiied portion of the extracted DNA is
cut using primers and reverse primers. These pieces which contain the segment of
interest are amplified by a technique cilled polymerase chain reaction (PCR).
Two techniques are available to separate the three genotypes. One is called
oligonucleotide hybridization where two specific probes are given the
opportunity to bind to the PCR-amplified product. One probe binds to the mutant
gene and the other to the normal. Both probes bind the heterozygote sequences.
The probes are labelled so the results can be read off autoradiographs. In the
second procedure the amplified DNA is incubated with a restriction enzyme that
will clea’ve copies of the defective DNA but not the normal. The digested DNA is
then separated by electrophoresis. In this process, fragments of smaller size
move greater and longer pieces move lesser distances. Staining permits
interpretation of the three possible banding patterns. Stress animals will show
two small fragments, normal, NN animals will show only one large fragment and
the Nn genotypes will be characterized by fragments of three sizes.
The diagnostic test has been
patented internationally by the Universities of Toronto and Guelph and licenses
are required for its use. These licenses may be obtained through Innovations
Foundation, University of Toronto, 203 College Street, Suite 205, Toronto,
Ontario, MST 1P9 Canada, (telephone 416-978-5117). The testing service is
offered at $50 per sample through either Dr. P. J. O'Brien, Pathology
Department, University of Guelph or through Dr. Charles Louis, Department of
Veterinary Pathobiology, University of Minnesota,St. Paul, MN 55108 (telephone
612-624-4202).
Industry
Application
At first glance it would appear
that removal of the PSS gene from the pig population should be the immediate
goal of the industry. That is probably the wise i. U.S. and Engbsh studies agree
that the nn sow farrows and raises about one pig per litter less than NN females
and that they produce litters that weigh about 10% less at weaning. Furthermore,
these animals are somewhat smaller at birth and at early ages (table 15), grow
no Inter and perhaps slower postweaning and produce shorter carcasses that are
almast always PSE. In Iowa State studies over 95% of the an genotype was PSE and
in Danish studies exactly 90% were either PSE or doubtful. In a large number of
ISU studies intransit death losses of PSS positive arumals have exceeded 15%.
The merit of these animals xeides in the fact that their ban composition exceeds
that of their normal NN littermates by 2.74% (table 17) ~ result of their
greater leanness particularly over their loin musde, and their greater Join
musde area and larger ham development (table 16). They also tend to consume leas
per day and be more efficient converters of feed to lean.
.Considerable interest has been
generated in deaignillg a breeding program that produces either all or a high
percentage of carriers. U the arw herd or breoh contzibuting to the production
of commercial sow are tested free of the disorder they could be mated to nn or
Nn boars and produce only NN or Nn pigs. None of the offspring should die.
stress deaths and any advantage of the camer genotype would be achieved. Studiea
dearly deraomtrate the Nn genotype to be intermediate between the homozygotes
for lean compositioa. Hence a pig crop made up of all carrie could be increased
in lean content by as much as 2% if stress boars were ased and by 1% if camer
sixes are used (table17). Why should one not try this approach? The answer
relates to the degree to which the carrier animal is misceptible to tbe
production of PSE meat. Results presented in tablts18-20providesome insight into
this question. Several studies confirm that the 45 min pH of carrier animals
averages about 6.1. Judge {1991) suggests a value in excess of 6.3 is necessary
for desirable musde quality, and that values of 5.8 to 6.3 produce intermediate
values. Values below 5.8 are certain to produce carcasses with severe PSE. The
reflectance values for the various genotypes presented in table19 nohow the
intermediate status of the Nn group. Slightly less than half of these carcasses
were considered to be of inferior rhuscle quality. Values in table20 are means
of a test that measures the degree the muscle proteins are denatured. Average
values of the Nn class are significantly below those for the nn dass but
somewhat higher than the NN group in at least the large study conducted in 1985.
In their large comprehensive study of the three genotypes the Danes found 27% of
the Nn genotype to be PSE and an additional 11% fell into the doubtful rangt.
Sixty-three percent of the earner group was reported to have produced normal
muscle quality.
It seems obvious that one should be
cautious about using the stress gene in any way. Its elimination from the pig
population is advisable. In the short term producing carriers could have some
merit since the resulting offspring will not die of stress and packers are not
yet in a position to detect PSE at a time when carcasses are still traceable to
their source. It clearly represents a "quick fix" to increase carcass leanness.
Perhaps new methods of ante-mortem swine handling and post-mortem carcass
handling will eliminate PSE produced by carcasses with intermediate 45 min
muscle pH values. But,until such time it would seem to be in the best interest
of the industry to eliminate the gene and concentrate on improving leanness by
direct selection in populations free of this disorder. Perhaps molecular
genetics procedures will one day separate the good and bad aspects of the gene
so the benefits can be realized without endangering the product quality of our
valuable industry.
Table 15.
Performance traits pigs by stress genotype.
Item |
Genotype |
NN |
Nn |
nn |
No. |
60 |
107 |
53 |
Birth
weight, kg |
1.31 |
1.35 |
1.28 |
21-day
weight, kg |
5.64 |
5.82 |
5.0a |
42-day
weight, kg |
11.5 |
11.8 |
9.6a |
Feed
consumption, kg/d |
2.73 |
2.60 |
2.60 |
Feed
efficiency, kg |
3.60 |
3.50 |
3.45 |
Average daily gain. Kg/d |
768 |
768 |
777 |
adiffers
significantly from other genotypes.
Table
16. Oueass traiD of pigs by stress genotype.
Item |
Genotype |
NN |
Nn |
nn |
No. |
52 |
84 |
32 |
Length, mm |
78.2 |
78.0 |
77.5 |
10th
rib backfat, mm |
39.1 |
36.8 |
36.8 |
Avg.
backfat, mm |
39.9 |
39.1 |
41.1 |
Loin
eye area, cm2 |
35.7 |
36.8 |
40.0a |
Dressing percentage |
73.6 |
74.1 |
74.7b |
adiffers
significantly from other genotypes.
bdiffers
significantly from the NN genotype
Table 17. Percentage differences in
separable lean between genotypes in Iowa State and Danish studies.
|
Genotype |
|
nn-NN |
Nn-NN |
ISU |
2.7 |
1.1 |
DMRI |
4.0 |
1.7 |
ISU = Iowa
State University (N= 28, 32 and 23 for NN, Nn and nn genotypes)
DMR = Danish
Meat Research Institute (N = 37, 95 and 70 for NN, Nn and nn genotypes)
Table 18.
Forty-five minute muscle pH in five studies by stress genotype.
Study |
Genotype |
NN |
Nn |
nn |
Christian, 1981a |
6.42 |
6.15 |
5.73 |
Denmark, 1985b |
6.16 |
5.90 |
5.35 |
Skaggs, 1990c |
6.39 |
6.16 |
5.80 |
Piedrafita, 1991d |
6.55 |
6.28 |
5.75 |
Hawkins, 1992e |
6.27 |
6.07 |
5.64 |
a
52, 84 and.32 pigs of each genotype, respectively
b
61, 167 and 111 pigs of each genotype, respectively
c
32 pigs of each genotype
d
16 pigs of each genotype
e
12 pigs of each genotype
Table 19.
Muscle reflectance measurements of loin muscle by genotype in four ISU studies.
Study |
Genotype |
NN |
Nn |
nn |
Christian |
22.5 |
24.6 |
29.0 |
Skaggs |
22.2 |
25.2 |
27.4 |
Piedrafita |
19.7 |
21.1 |
31.8 |
Hawkins |
23.0 |
25.5 |
30.2 |
Table
22. Average percent heterosis for various swine traits.
Trait |
First
cross using Purebred female |
Multiple Cross using Cross bred female |
Litter
size born alive |
1.0 |
5.6 |
Litter
size at 21 days |
8.0 |
17.1 |
Lither
weight at 21 days |
11.3 |
24.3 |
Litter
weight at 21 days per female exposed |
11.3 |
29.0 |
Days
to 220 pounds |
6.5 |
7.7 |
Average daily gain |
9.4 |
9.4 |
Peed
per pound gain |
2.3 |
2.3 |
The two-breed rotational scheme is
still being used by a number of producers. A York x Duroc or York x Hampshire
scheme is still very popular. These breeds complement one another well and
generaQy produce a final product with good packer acceptance. The disadvantage
of the two-breed system is that only 67% of maximum heterosis is achieved after
six generations. One of its advantages is simplicity.
To capitalize on heterosis, the
addition of a third breed increases heterosis to 86% of maximum. This is the
most popular system with producers. It permits choice of breeds for maximum
complementation. The Hampshire, Duroc and Yorkshire breeds are those most
commonly used; the Hampshire for carcass desirability, the Duroc for superior
growth and efficiency and the Yorkshire for maternal performance.
There are several disadvantages of
the three breed rotational system. The genetic makeup of the sow herd is
constantly undergoing change and boars of all three breeds must be kept w
inventory and properly mated if maximum heterosis is to be realized. Because of
the genetic diversity, there is a great deal of variation in the output. The
second limitation is that the system doesn’t permit utilizing each breeds
attributes to the best advantage. Ideally, oui producers would like to maximize
the Hampshire carcass desirability in all market pigs rather than in one third
of them.
Static or terminal systems of
mating are becoming more popular since larger complexes. permit specific mating
schemes for intraherd replacement gilt production and since Fl gilts art
available from several commercial companies. These systems allow the positioning
of brea where its best characteristics may be more fully realized and where it
permits pigs of consistent breed background to be produced over a long period of
time. Perhaps most importantly, the static cross can be designed to achieve 100%
of maximum heterosis in both th~ pig and the dam. The most popular breed choices
for this system have been a female produced from the mating of two white breeds.
This F1 female is generally mated to Duroc, Hampshire Spotted or hybrid boars.
Table 20. Percentage of light
transmittance of loin muscle by genotype in three
ISU studies.
|
Genotype |
Study |
NN |
Nn |
nn |
Christian |
24.3 |
33.5 |
73.0 |
Skaggs |
20.5 |
24.2 |
59.0 |
Piedrafita |
5.6 |
8.9 |
71.1 |
Reduction in subcutaneous and
intramuscular fat (marbling) has definite benefits in improving the image of
pork in the eyes of the consumer. However, lowering marbling to values below
2.5% may result in reduced juiciness and flavor. Repartitioning agents will also
reduce marbling, and hence, methods to detect marbling and other muscle quality
traits in the live pig would be highly desirable. A shot biopsy procedure
developed in Germany and Czechoslovakia shows promise for this purpose.
Crossing
Systems
Considerable variation exists in
crossbreeding systems employed by commercial producers (Table 21). Most are
designed to capitalize on heterosis present in most traits (Table 22). Although
most operations use some type of crossing scheme, variations in the breeds used
and the crossing method are common.
Table 21. Type
of Crossbreeding System used by Percentage of Farms*.
System |
Percent of Farms |
2-breed rotational |
14.8 |
3-breed rotational |
44.8 |
4-breed rotational |
10.5 |
Rotaterminal 2-breed |
5.3 |
Rotaterminal 3-breed |
5.8 |
Terminal |
18.8 |
|
100.0 |
*Michigan
State University Survey 1988
A compromise system of mating that
achieves the advantage of a specialized maternal female and 100% of maximum
heterosis in the final pig produced is what has become known as the
static-rotational or rotaterminal system. In this system the best producing sows
from within the herd are identified and mated to boars of the maternal breeds
(usually Yorkshire, Landrace or Chester Whites) to produce the replacement
females. The balance of the sow herd is mated to a terminal sire and all of the
offspring marketed. If 20% of the litters are sired by maternal line boars in a
3-breed rotational system, a replacement rate of one female from each such
litter will allow the sow herd to remain of constant size if approximately 20%
of the sows are culled twice each year.
The main advantages of this system
are intraherd replacement of females, selection of female replacements from the
best producing sows and nearly optimum heterosis (100% in the pig and 86% in the
sow). Because smaller herds do not maximize use of maternal line boars,
artificial insemination offers an excellent alternative in this system.
Summary
Genetic improvement in commercial
swine herds depends upon selection and optimum heterosis. Since most genes come
from outside the herd, accurate and effective selection both within and across
herds by the seedstock supplier is crucial if his commercial clients are to
improve at an optimum rate. Participation in selection programs utilizing EPDs,
such as STAGE 6, permit seedstock producers to maximize the rate of genetic
change when results are used in choosing replacements and emphasis is placed on
the economically important traits. Genetically linked herds provide small
seedstock herds with a collective population size large enough for effective
selection programs. This is particularly true for SPI traits. AI offers great
potential to strengthen genetic ties between herds, facilitate across herd
selection and transfer of genes between herds. This approach can permit the
traditional seedstock producer to compete effectively with larger herds or
breeding companies that have the advantage of large herd size. Survival of the
small private purebred breeder could very well depend on adoption of these
approaches.
The commercial producer can select
outstanding animals of superior pure lines of breeds and combine them into
programs that capitalize on heterosis. The optimum utilization of healthy,
genetically superior stock that is well-managed, and fed to achieve their
genetic potential, should result in the low cost production of a superior pork
product.
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Bereskin, B. and
R. J. Davey. 1976. Breed, Line, Sex and Diet Effects and Interactions in Swine
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